Kinsey’s reports were based on careful interviewing but on volunteers, prisoners and other non-random groups. Why does that make their prevalence estimates unreliable even though the interviews were well conducted?

B. Interview quality shapes reports, but sampling decides if results represent a population

Careful interviewing improves the accuracy of each answer and does nothing about the frame. If the people interviewed differ systematically from the population, the resulting percentages describe the volunteers, and no amount of interviewing skill converts them into national figures. The same distinction separates sampling bias from measurement error everywhere in the discipline.